1. Field of the Invention
The present invention relates to a substrate dryer, and more particularly, to a substrate dryer in which the concentration of isopropyl alcohol supplied for drying a substrate can be increased
2. Description of the Related Art
Generally, to fabricate semiconductor devices, a lithography process, a deposition process, and an etching process are repeatedly performed on a substrate. In these processes, contaminants such as particles, metals, natural oxides, organic layers, etc. may be left on the substrate. Therefore, a cleaning process is performed on the substrate to remove these contaminants. Then a drying process is performed to remove the remaining contaminants.
Conventional drying methods include a spin drying method, an isopropyl alcohol (IPA) vapor drying method, an isopropyl alcohol Marangoni drying method, and the like.
Regarding the spin drying method, a drying apparatus is relatively simple and cheap, though it is difficult to continuously perform a cleaning process and a drying process. It is also impossible to completely dry the substrate.
Regarding the IPA vapor drying method, since a drying process is performed in IPA vapor, that is, in a clean space, less contaminants are generated. However, problems arise because a particular bath is required for the cleaning and drying processes and water spots are typically generated due to exposure of the substrate to oxygen when the substrate is transferred to a drying bath from a cleaning bath.
Regarding the IPA Marangoni drying method, the substrate is dried using a Marangoni effect by vertically drawing up the substrate from a deionized (DI) water bath, or draining the DI water bath while holding the substrate fixed, and blowing nitrogen gas and IPA at the surface of the substrate near the air/liquid interface. In the IPA Marangoni drying method, it is possible to drastically reduce the amount of IPA and to prevent water-spot occurrence due to exposure of the substrate to the air because the substrate is not transferred from a cleaning bath to a drying bath.
On the other hand, when the substrate is exposed to oxygen after cleaning, unnecessary oxides are generated on the substrate and then such oxides are left as residues, thereby generating water spots. When the substrate is exposed to nitrogen, water spots are generated, which have a size smaller than when it is exposed to oxygen. These water spots are generated when hydrophobic membranes such as poly-silicon, oxide, and nitride and hydrophilic membranes exist together in an active region of a semiconductor device have various step-difference structures.
However, in a recent semiconductor process requiring a high integration density corresponding to a process margin of 0.11 micrometers or less, in a substrate with a large aperture (for example, 300 mm), patterns include both simple structures and more complicated three-dimensional structures. As an example, with a RCAT (Recess Channel Array Transistor) process, which may be applied to improve refresh characteristics of transistors of high-integrated semiconductor devices, an active region is formed in a trench pattern, and hydrophobic and hydrophilic membranes exist together. In addition, the active region is formed to have a step-difference of about 1500 Å. In such a deep trench pattern, it is difficult to completely remove leftover water while drying the substrate.
As mentioned above, because such a pattern structure is considerably challenging to the success of a drying process, water spots may occur. Particularly, water spots occur intensively in the lower part of the substrate. Further, these water spots not only may act as a mask in a subsequent etching process, so that an etching residue may occur, but also are obstacles to an epitaxial process such as chemical vapor deposition (CVD) and epitaxial growth.
In the IPA Marangoni drying method performed in the nitrogen atmosphere, water spots are caused by the presence of valleys or holes and by water residue, which results when the substrate is directly exposed to the nitrogen atmosphere after being in DI water, and before going through IPA during the progression of the Marangoni effect.
Among dryers employing a conventional IPA Marangoni drying process, for example, ones manufactured by Korea DNS. Co. Ltd, and YieldUP International of U.S.A., the concentration of IPA is in the range of 0.15 to 0.25%. Compare this to when the concentration of IPA is in the range of about 2 to 3% and water spots do not occur during the drying process.
On the other hand, in the IPA Marangoni dryer, as disclosed in U.S. Pat. No. 6,598,312, IPA is supplied to the inside of a cleaning bath through an IPA bubbling device, and IPA bubbles are generated by supplying nitrogen to the inside of a bubble tank.
As a result, it is possible to increase the concentration of IPA to some level by increasing the amount of nitrogen to be supplied, or by raising the pressure of nitrogen to be supplied. However, there is a problem that if the amount of nitrogen is increased over the reference level of a particular process, then the mixture ratio of nitrogen and IPA is not within a regulated level, so that unexpected water spots can be generated due to the excessive amount and pressure of nitrogen.